How it is possible to build 3 undergrounds upon each other in that manner for me that let them be in service under each other
With latest versions including new underground view, you can see underground level separately .
(Moved to help requests. Please don't put topics in General Discussion if there is a suitable board for them.)
Gauthier refers to the nightly builds, but those might not be suitable for (long-term) playing.
Not using nightlies, you can open three tunnels at different heights for each of the levels. If the stations at each level are on the same vertical line, there will be the same station and passengers will transfer among different levels.